Physical properties of the mitral valve tissue assessed by tissue sound speed in cardiac amyloidosis: relationship to the severity of mitral regurgitation

Cardiac amyloidosis has been documented to show mitral regurgitation (MR) and a thickened mitral valve (MV) due to amyloid deposits. However, the changes in the physical properties of the thickened MV tissue in cardiac amyloidosis, which may be a causative factor of the MR, have not been described. Physical properties of the tissue, which are expressed by the elastic bulk modulus, can be evaluated by tissue sound speed. If biological tissue is assumed to be fluid-like, the tissue sound speed may be given by c= square root of K/rho, where c is the tissue sound speed, K is the elastic bulk modulus, and rho is the density. A reduction in tissue sound speed indicates a reduction in the elastic bulk modulus of the tissue, assuming that there is little change in rho. This suggests that the tissue is less elastic. The purpose of this study was to assess the physical properties of MV tissue by evaluating the sound speed of the MV tissue in cardiac amyloidosis. MV specimens were obtained at autopsy from 20 control adults without cardiovascular diseases and from 20 patients with cardiac amyloidosis. An acoustic microscope operating at 450 MHz was used to measure the tissue sound speed in the tip and basal portions of the MV tissue. The density of MV tissue was measured by microgravimetry. The severity of the MR had been evaluated by Doppler echocardiography before death, and it was compared with the tissue sound speed measured after death. In cardiac amyloidosis showing mild MR, the tissue sound speed of the MV in the tip portion (1605 +/- 19 m/s) and in the basal portion (1791 +/- 64 m/s) were lower than the corresponding values in control subjects (1637 +/- 42 m/s and 1851 +/- 62 m/s). However, these differences were not statistically significant. In cardiac amyloidosis showing moderate MR, the tissue sound speed of MV in the tip portion (1563 +/- 17 m/s) and in the basal portion (1654 +/- 59 m/s) were significantly lower than the corresponding values in the control subjects (p < 0.001) and the patients with mild MR (p < 0.05). No significant differences were observed in the density of MV tissue among the three groups. Therefore, the low value of the MV tissue sound speed in patients with cardiac amyloidosis indicated a reduced elastic bulk modulus, suggesting the less elasticity of the MV tissue. Furthermore, the patients with moderate MR demonstrated the greater reduction in the tissue sound speed than the patients with mild MR. The data suggest that the changes in physical properties of the MV tissue may be one of the causes of MR in cardiac amyloidosis.